Send packets only to players within tracking range. Fixes #860, #1114

This fixes a bug introduced by spigot's optimization to entity tracking
ranges. Mods such as Thaumcraft were able to send packets to players
outside of configured tracking ranges. To ensure this does not happen,
tracking ranges are now validated before attempting to send a packet to a
player.

* Fixed setup.py failing due to invalid md5 for 1.6 server jar.
* Updated README
Revert "Add check and message for Java 8 -- thanks @agaricus"

This reverts commit 6daf54d35bb0f56ffa5ccaecb7d6d4bd6f801d6c.